In recent years, the heat insulation of residential buildings has become increasingly important for economic and ecological reasons. This is also clearly expressed by the more stringent heat insulation regulation (WSVO III) in force since 1995. There are two principal heat insulation systems: heat insulation renders (e.g. according to DIN 18550) and composite heat insulation systems (e.g. according to UEATc guideline and Austrian standard B6110).
Advantages of composite heat insulation systems (CHI systems) have been and are being constantly expressed in numerous publications (for example in the information brochures of the Fachverband Wxc3xa4rmedxc3xa4mm-Verbundsysteme e.V.).
The structure has essentially been established and comprises the parts consisting of insulation panel for heat insulation, adhesive for fastening the insulation panel to the substrate, reinforcing layer for compensating tension and decorative plaster for protection from weathering (cf. for example Das Deutsche Malerblatt [German Painters"" Newssheet] December 1996, pp. 32 et seq.). Materials for insulation panels are primarily those based on mineral fibers or polystyrene.
The insulation panels are fastened either by means of a ready-to-process dispersion adhesive, a dispersion adhesive with added cement or a cement-bonded ready-made mortar to which dispersion powder has been added. Heat insulation panels based on polystyrene and also mineral fiber panels are generally hydrophobic. However, a cement-bonded, unmodified adhesive mortar does not adhere to such hydrophobic substrates. In order, however, to ensure sufficiently strong bonding of the CHI system, minimum tensile adhesive strengths are required in the abovementioned UEATc guideline and in Austrian standard B6110.
When such ready-made mortars are used as an adhesive and/or reinforcing layer, it is therefore essential to use dispersion powders since otherwise subsequent adhesion of the adhesive to the insulation material is not obtained. However, in this case too, the improved adhesion of the adhesive, produced by the dispersion powder, is often not sufficient to meet the requirements, in particular with respect to the tensile adhesive strengths after wet storage as required in the guidelines.
The mode of action and basic composition of dispersion powders are known and described in many publications. Only the references J. Schulze IZ 9 (1985), 3-14 and G. Schultz, VDI Berichte [VDI Reports] 384 (1980), 25-37 are mentioned here as examples.
The increase in the tensile adhesive strength also plays an important role on nonhydrophobic substrates. Thus, a polyvinyl alcohol soluble in cold water and comprising amino-functional vinyl units is described, for example in EP-A-0 532 096, as a component of a redispersible dispersion powder which improves the tensile adhesive strength on mineral substrates, such as, for example, sidwalk slabs.
The preparation of polyvinyl acetal is known from the prior art. Usually, polyvinyl alcohol is used as a starting material and is reacted with aldehydes in aqueous solution under acid catalysis to give polyvinyl acetal. The reaction is usually carried out to complete, theoretical conversion of all hydroxyfunctionalities, with the result that the polyvinyl acetal obtained therefrom is water-insoluble and is precipitated.
EP-A-0 594 026 describes a polyvinyl acetal which is soluble or at least dispersible in water even at high degrees of acetalization, as a result of the incorporation of comonomer units carrying sulfonate groups. In addition, this publication also describes the isolation of polyvinyl acetal by freeze-drying or spray-drying. However, this dry polyvinyl acetal is not a dispersion powder in the traditional sense as described in the abovementioned citations.
The use of polyvinyl acetal as a protective colloid in emulsion polymerization is also known. Thus, DE-C-936 063 describes the acetalization of polyvinyl alcohol used as a protective colloid, during the emulsion polymerization of vinyl acetate, by addition of aldehydes and acids during the actual polymerization. Dispersions obtained therefrom are evidently distinguished by good water resistance.
DE-C-1 169 670 states that corresponding water resistance of dispersions are obtained by the use of a graft polymer comprising polyethylene glycol and vinyl acetate. In some examples, the use of partially acetalized polyvinyl alcohols is also mentioned. In contrast to DE-C-936 063, DE-C-1 260 791 describes the use of acetalized polyvinyl alcohols based on monounsaturated aldehydes. Evidently, the viscosity of the dispersions to be prepared can be influenced in the desired manner by the use of unsaturated aldehydes in direct comparison to the saturated aldehydes.
DD-A-222 880 describes the use of partially acetalized polyvinyl alcohols in emulsion polymerization. Here too, the water resistance of the films is of primary importance.
Finally, DD-A-251 560 discloses a selection of partially acetalized polyvinyl alcohols which are distinguished by a minimum surface tension of corresponding solutions.
However, it is known that acetals tend to undergo cleavage at low pH. This is also utilized in particular in chemistry involving protective groups. Since, in all examples of the abovementioned documents, either an acidic buffer having a pH of less than 4 is used during the polymerization or the use of persulfates as initiators permits a corresponding pH, as is usual in the polymerization of vinyl acetate-containing dispersions, elimination of the acetal groups is very highly probable. Consequently, it is very improbable that partially acetalized polyvinyl alcohol is still present after the end of the polymerization.
The question furthermore arises as to whether the protective colloid effect described can be achieved at all in the presence of partially acetalized polyvinyl acetal. In fact, the polymerizations always take place at temperatures of more than 60xc2x0 C., in general even more than 70xc2x0 C. However, partially acetalized polyvinyl alcohols have a water solubility which decreases with the increase in degree of acetalization. Thus, for example, in the case of n-butanal as aldehyde, the partially acetalized polyvinyl alcohol is precipitated from the aqueous solution already at below 60xc2x0 C. or even at a very low degree of acetalization of 8.5%, which corresponds to 6.6% by weight at a 98% degree of hydrolysis of the polyvinyl alcohol used. At a theoretical degree of acetalization of 13.9% (this corresponds to 10.8% by weight at a degree of hydrolysis of 98%), the polymer is precipitated from the solution even at about 30xc2x0 C.
It was the object of the present invention to develop dispersion powders which improve the tensile adhesive strength of cement-bonded ready-made mortars modified therewith, in particular after wet storage on hydrophobic substrates, in particular polystyrene.
Surprisingly, it was found that, on adding partially acetalized but still water-soluble polyvinyl alcohols to the dispersion, spraying gives dispersion powders which ensure improved wet adhesion of the ready-made mortars described above.
The present invention relates to dispersion powders comprising partially acetalized, water-soluble polyvinyl alcohols which lead to better adhesion of correspondingly modified cement-containing materials to hydrophobic substrates, the use of these, and process for their preparation.
The present invention thus relates to a dispersion powder comprising
a) at least one base polymer from the group consisting of the vinyl ester, vinyl ester comonomer, vinyl ester/ethylene, (meth)acrylate and styrene/acrylate polymers,
b) from 1 to 25% by weight, based on the base polymer, of at least one protective colloid,
c) from 1 to 25% by weight, based on the base polymer, of at least one partially acetalized, water-soluble polyvinyl alcohol,
d) from 0 to 20% by weight, based on the total polymer weight, of anticaking agent and
e) if required, further additives.